The simultaneous contrast paradigm was used to evaluate responsiveness to a low (0.05 M) and a high (0.5 M) concentration of sucrose under two conditions in intact and chronic decerebrate rats. In one condition the low concentration was presented on one day and the high concentration on another. In the other condition presentation of the two concentration was alternated within the same daily session. In each case there was a total of 40 trials/day during which the stimulus was delivered intraorally for 2 s at a rate of 1.5 ml/min with a 30-s intertrial interval. The results showed that the intact rats always licked more for the high than for the low concentration of sucrose but that the magnitude of the effect was larger when given the opportunity to compare the two concentrations within the same daily session. The decerebrate rats produced a similar pattern, but the concentration effect was evident only when the stimuli were alternated within the same daily session. These data stand as the first evidence that the isolated caudal brain stem is adequate for the expression of a behavior that depends on comparison processes involving short-term memory.